The hypothesis that an important function of the perceptual machinery is to construct an analogue model of the tridimensional world has considerable plausibility, but has received little systematic attention in recent years. The proposed research will look for certain effects that may reasonably be expected on the basis of this hypothesis, but not otherwise. Three major questions, all involving spatiotemporal relationships among stimuli, will be investigated: 1. Does the interstimulus interval yielding optimal apparent movement between visual stimuli vary with the proximal or the distal separation between stimuli? 2. Can apparent movement have a masking effect on (a) a flash of light, or (b) a movement, occurring in its path? 3. When two brief stimuli are presented in succession to different modalities, does the distal separation between stimuli (under conditions of invariant proximal stimulation) significantly affect (a) the interstimulus interval associated with optimal impressions of movement, or (b) the threshold for successiveness of the stimuli? It is hoped that these studies will contribute substantially to the understanding of space perception and spatial representation.